What is dielectric cure monitoring (DEA)?
Dielectric cure monitoring, also known as Dielectric Analysis (DEA) is the only method that can measure thermoset or composite cure state in real time under actual process conditions. Consequently, DEA can generate results in the laboratory that are directly applicable in manufacturing. In brief, DEA uses an AC signal to measure a thermoset’s resistivity and permittivity, which are the material’s dielectric properties. Resistivity itself has a frequency independent component due to the flow of mobile ions and a frequency dependent component due to the rotation of stationary dipoles.
Although often called DC resistivity for brevity, frequency independent resistivity actually extends across a range of frequencies that includes DC (0 Hz). Frequency independent resistivity—more commonly called ion viscosity (IV)—correlates with cure state, making it a useful material probe. By taking advantage of optimal frequencies, AC measurements—unlike DC methods—can deal with distortion of data caused by electrode polarization and can work through release films and vacuum bags.
In many cases the change of log(IV) is proportional to the change of mechanical viscosity before gelation and proportional to the change of modulus after gelation. Consequently, DEA can observe cure throughout a process.
The figure above illustrates the relationship between ion viscosity and mechanical viscosity for a curing thermoset with one temperature ramp and hold. At first, as temperature increases, ion viscosity decreases because the thermoset becomes more fluid and therefore less resistive. The reaction rate increases as the material becomes hotter.
At some time, the increase in ion viscosity due to polymerization overcomes the decrease in ion viscosity due to rising temperature. This point is the ion viscosity minimum, which also occurs at about the time of minimum mechanical viscosity.
After the minimum, ion viscosity increases as the reaction accelerates and the material becomes more viscous. At gelation, with the start of crosslinking and network formation, the mechanical viscosity increases rapidly until it is unmeasurable. Ion viscosity, however, continues to provide information about cure state past the gel point.
As the concentration of unreacted monomers diminishes and crosslinking becomes more extensive, the reaction rate decreases; consequently, the slope of ion viscosity also decreases and eventually ion viscosity will have zero slope when cure has stopped completely.